FIG. 15 shows an apparatus heretofore proposed for electrically treating sewage (Unexamined Japanese Patent Publication SHO 59-87093).
The treating apparatus comprises a treating tank 100 made of insulating material and having an influent pipe 105 connected to its lower portion and an effluent pipe 106 connected to its upper portion, and two electrode plates 101, 102 formed with a multiplicity of holes 103 and arranged one above the other within the tank 100 with a spacing provided therebetween. Current is passed between the lower electrode plate 101 serving as a positive electrode and the upper electrode plate 102 as a negative electrode. The electrode plates define an electrolytic chamber therebetween which is packed with a material 107 in the form of a multiplicity of spherical pieces. These pieces are made of a soluble metal, such as an aluminum alloy, having a great ionization tendency.
Sewage containing an additive, such as aluminum chloride, added thereto for ensuring a promoted reaction is supplied to the treating tank 100 through the influent pipe 105. The sewage flows into the electrolytic chamber through the lower electrode plate 101 and moves upward through the clearances between the packed pieces 107 in contact therewith.
While the sewage flows upward through the clearances between the packed pieces 107, the contaminants in the sewage are electrically neutralized and coagulate on separating out. On the other hand, the metal ion dissolves out from the surface of the packed material 107 and reacts with chlorine ion or the like in the sewage to form aluminum hydroxide. The aluminum hydroxide is held to the coagulation through a covalent bond, adsorbed thereby or enclosed therein, gradually forming insoluble flocs. These flocs adsorb hydrogen gas evolved at the negative electrode and oxygen gas produced at the positive electrode, become decreased in apparent specific gravity and float at the upper portion of the treating tank 100.
The contaminants floating in the form of flocs (hereinafter referred to as "scum") flow out through the effluent pipe 106 along with water, forming two layers therewith, and are supplied to a known floatation separation tank (not shown), in which the scum is removed.
The above apparatus further converts the contaminants to harmless and stable substances to achieve a reduction in BOD and COD through an oxidation reaction which occurs when the sewage passes through the positive electrode plate 101 and a reduction reaction which takes place when the sewage flows through the negative electrode plate 103.
However, the electrical sewage treating apparatus has the problem of being inefficient in removing some kinds of contaminants from sewage and failing to achieve a satisfactory removal efficiency in treating sewage containing many kinds of contaminants.